In the case of many products with a limited duration of life it is not possible, or barely possible, for the consumer to detect when said duration of life has come to an end and there is therefore no purpose in making further use of the product.
The end of said duration of life cannot be indicated by printing on a date during the production since the duration of life only begins at the moment when the consumer actually starts to use the product. In such cases there is a need for an indicator system which is activated at the moment the product is put into use, and which undergoes a clearly visible change at the end of the duration of life.
Other products have a limited durability from the moment of production. In these cases a final date of use is generally indicated on the packaging. Said date is often difficult for the consumer to find or poorly legible. In these cases too, an eye-catching indicator could be used with advantage.
Typical examples of products with a limited duration of life from the moment they are put into use by the consumer are so-called air-treatment products. Such products are intended to disperse one or more volatile components, such as perfumes, deodorant components, insecticides, pheromones or repellants in the atmosphere by evaporation. If perfumes or deodorant components are involved, said air-treatment products are usually termed air fresheners.
Indicator systems specially intended for airfreshener systems are, for example, described in the U.S. Pat. No. 4,128,508 and the Japanese Patent applications Nos. 56,053,461 and 56,131,681. These are based on the change in colour of a pH indicator combined with a slowly evaporating acid or base. Such systems have the disadvantage that the choice of suitable volatile acids and bases is limited, inter alia, because they often have a strong and usually undesirable smell of their own. Furthermore, many are not completely harmless to health and, in addition, volatile acids may have a corrosive effect on metals in the surroundings. Other colour indicator systems, which are described in the Japanese Patent application No. 55,027,200, are based on the slow oxidation or reduction of organic pigments by added oxidation or reduction agents respectively, or by atmospheric oxygen. Such systems have hitherto only proved suitable for adding to, for example, shampoo, depilitories, toothpastes and the like in order to indicate action times from a few minutes to a few tens of minutes. For products with a duration of life of a few days to a few months, such systems are completely unsuitable.